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while playing with stuff I hacked together a very small plugin implementing the World Magnetic Model published by NOAA into OpenCPN.
The plugin requires OpenCPN 2.4.324 or later to work correctly.
In the preferences you can tailor the amount of displayed information to your liking - from one-line declination display at boat's location to all the components that please the freaky magnetician.
Useful? And yes, I know I should have done it as an instrument in the dashboard plugin

I was wrong about the annual change. It only shows the variation, both in decimal degrees and in degrees and minutes. However, the annual change in variation is also availbale in the model, as stated on the NOAA site.

It is necessary to show the latest date that the data in the model is still valid (or the year for which the data is valid AND the annual change)! Also the units used for presentation of the data should be shown (nT -nanoTesla, deg - degrees, min - minutes per year).

Anyway, a very handy tool.

Crashing seems to happen whenever a presentation change is made without switching WMM off first, using the icon.

NAV,
the chart is not part of the plugin, it is georeferenced and available for download from opencpn.org for quite some time.
Regarding the terminology I stayed with all terms magnetic to keep it consistent. I'm personally not that allergic to declination as my book said it's a synonym to variation when I last read it
Crash reproduced - can happen only when you change prefs before displaying the plugin window for the first time - fixed. Will release new version when time comes.

Nice plugin!
I am allergic to "landlubber" terminology.
Let a future astro-navigation plugin use "declination", and please use "variation" which is the traditional nautical term.

Yeah, I had the crash as well.

I would like to compile he plugin for my 64 bit Ubuntu, but get a bit confused as " git clone git://github.com/nohal/OpenCPN.git " don't include this plugin. Is there an other git command I should use?

Changed window behavior from being globally on top to be just on top of the application window

Night mode support

Units shown (if you don't like it, make your custom .mo file)

Model expiration note and instructions added in plugin description

Plugin now clearly shows it's hate to the magnetician community by strictly using the nautical term Magnetic Variation instead of Geomagnetic Declination The magneticians can also make their own .mo to revert it as a revenge.

World Magnetic Model PlugIn for OpenCPN\n
Implements the NOAA World Magnetic Model\n
More information: http://www.ngdc.noaa.gov/geomag/WMM/\n
The bundled WMM2010 model expires on December 31, 2014.\n
After then, if new version of the plugin will not be released\n
in time, get a new WMM.COF from NOAA and place it to the\n
location you can find in the OpenCPN logfile.